1. Field of the Invention
The invention relates to a system and a method for determining the orientation of fibers in fibrous material webs, particularly paper webs.
2. Discussion of Background Information
The article xe2x80x9cHigh-Resolution Fibre Orientation and Basis Weight Measurementxe2x80x9d by B. Drouin et al. in the Journal of Pulp and Paper Science, Vol. 22, No. 7, Jul. 7, 1996 mentions an instrument with which the fiber orientation in paper is measured by a transmission measurement with a rotating plane of polarization. The instrument is based on filtered black body radiation in the far infrared region (FIR) of the electromagnetic spectrum. Some disadvantages of this conventional device are its high cost and the fact that it works with only one wavelength.
The invention provides for a system and a method of the type initially described, in which the fiber orientation can be determined more efficiently and with high precision.
The invention utilizes at least one source of electromagnetic radiation disposed on one side of a fibrous material web and at least one sensor for receiving the radiation emitted by the source disposed on the other side. The radiation from the source penetrates the fibrous material from one side and is sensed on the other side. Moreover, at least one optical device, for influencing the propagation of the radiation as a function of its polarization properties, can be positioned in the path of the radiation, e.g., between the source and the sensor.
It has been discovered that the interaction between electromagnetic radiation and fibrous material webs having a homogeneous or at least dominant or prevalent fiber orientation, can be used to obtain information from which the fiber orientation can be determined. In principle, any desired wavelengths of the radiation can be used. However, wavelengths in the realm of visible light and/or infrared radiation are preferred. Moreover, NIR (Near Infrared Radiation) is most preferred. Additionally, other wavelengths may be utilized or required at the same time so that the system can cope with the differing ash content and/or other variations of the paper properties.
The optical system of the invention utilizes radiation whose propagation has been influenced to produce linearly polarized radiation. The invention may use completely unpolarized radiation from various sources, such as a natural light source, partially unpolarized light, or unpolarized radiation. This radiation which is emitted from the source can then be linearly polarized prior to entering and/or interaction with the fibrous material web. In this case, the optical system then serves as a polarizer. Alternatively, the radiation which is emitted from the source can then be linearly polarized after passing through and/or interaction with the fibrous material web. In this case, the optical system is used as an analyzer. Such a design allows the direction of polarization of linearly polarized radiation to be detected after the radiation has passed through the fibrous material web.
The invention takes advantage of the fact that the propagation of linearly polarized radiation is influenced to the extent that the intensity of the linearly polarized radiation, that the optical device permits to pass, is a function of its direction of polarization.
In at least one embodiment of the invention, the optical system includes at least one polarizing filter. With filters of this type, it is possible both to produce linearly polarized radiation and to determine the polarization direction of linearly polarized radiation. Moreover, the use of such a polarizing filter permits the implementation of many different arrangements, all of which are characterized by comparatively simple construction in terms of measurement technology as well as a high degree of measuring precision.
Thus, it is possible to arrange a system having a single optical device in the form of a polarizing filter located between the source and the fibrous material web. The invention provides for the intensity of the linearly polarized radiation, which is produced by the polarizing filter and penetrates the fibrous material web, to be measured by sensor changes as a function of the orientation of the polarizing filter relative to the orientation of the fibers in the fibrous material web. Utilizing this technique, the fiber orientation can be determined in a comparatively simple manner by utilizing repeated measurements of intensity at different orientations of the polarizing filter relative to the fibrous material web. Such a system design can utilize a polarizing filter which is mounted such that it can rotate about an axis running perpendicular to the fibrous material web running direction.
The system functions as follows: It is assumed that a measured light intensity depends on the main fiber orientation in the paper and on the orientation of the polarization filters. The measured light intensity will be highest if the polarization filter and the fibers are oriented in the same direction (at 0xc2x0 and 180xc2x0). The measured light intensity will be lowest in the orthogonal directions (at 90xc2x0 and 270xc2x0). It is further assumed that the intensity distribution in polar coordinates has therefore about an elliptical shape: the largest diameter at 0xc2x0 (a), and the smallest diameter at 180xc2x0 (b).
If the difference axe2x88x92b is high, the fibers are very strongly oriented only in one direction. However, if the difference is small, there is only a very small orientation or an almost equal distribution of the fibers.
Accordingly, if the highest signal is attained, when the orientation of the polarization filter corresponds exactly to machine direction of the paper, the fiber orientation is 0xc2x0 relative to the machine direction. Otherwise, the fibers are not oriented properly in the machine direction. Acceptable or desired values are between approximately xe2x88x922xc2x0 and approximately +2xc2x0, while unacceptable or undesired values are larger than approximately 10xc2x0.
Thus, an algorithm may be utilized which has two parts: the data of the ellipse, and the relationship or how this data relates to paper properties. Accordingly, in order to calculate the orientation/shape of the ellipse, at least (3) three measurements have to be taken (three signals with polarizing filters in three different orientations). This can be performed using the usual quadratic equations which use the known properties of the ellipses.
In order to relate these data to the paper properties, empirical solutions are required. The signals a and b (and/or algorithmical combinations of these signals like axe2x88x92b, a+b, and a/b), and the orientation angle of the ellipses are compared to lab tests. Thus, one can use e.g., Least Squares methods like xe2x80x9cPartial Least Squaresxe2x80x9d (or other similar methods) in order to derive formulas to derive paper properties from the characteristics of the ellipse.
On the other hand, breaking load ellipses are something similar. Sample paper strips are typically taken having three different angles relative to the machine direction (i.e., xe2x88x9230xc2x0, 30xc2x0, and 0xc2x0). From these three measurement values, an ellipse is calculated. Accordingly, the ellipse reflects the strength properties of the paper. The strength is greatest in the direction of the main fiber orientation and lowest in a direction orthogonal to it. Thus, the strength properties in different angles serve as an indication of the fiber orientation. Unfortunately, this technique cannot be performed on line as it requires that the paper samples or sections be removed or cut from the web.
Accordingly, the system may utilize several fixed polarizing filters which have defined orientations or polarization directions when in the measurement position instead of one or more movable polarizing filters. This design allows the filters to be exchanged quickly. Moreover, the filters may be arranged in a configuration known as a filter wheel.
According to another variant of the invention, a single polarizing filter is positioned between the fibrous material web and the sensor with the source being located on the other side of the fibrous material web. This design or arrangement exploits the capability of a fibrous material web having an at least dominant or prevalent fiber orientation to linearly polarize incident radiation, at least to a small degree, by interacting with the incident radiation. The polarization direction dependent on the fiber orientation can be detected with the polarizing filter, which serves as an analyzer in this arrangement. In this arrangement, the fiber orientation can be detected by measuring the intensity of the radiation which is permitted to pass through the polarizing filter as a function of the orientation of the polarizing filter relative to the fibrous material web.
In yet another variant of the invention, at least one polarizing filter is utilized on each side the fibrous material web. In this arrangement, one polarizing filter located between the source and the web. This filter produces linearly polarized radiation from source radiation. Another filter is located between the web and a sensor device. This filter is used to detect the direction of polarization. In this arrangement, either of the two polarizing filters or both can be mounted such that they can rotate about an axis running perpendicular to a plane defined by the fibrous web running direction.
In another embodiment of the invention, a single polarizing filter is positioned between the source and one side of the fibrous material web. A plurality of polarizing filters are located on the other side of the fibrous material web. This arrangement also utilizes a sensor associated with each of these polarizing filters. Moreover, these filters can have different polarizing orientations relative to the fibrous material web travel direction. Additionally, in this design, because of the different orientations of the polarizing filters on the sensor side, none of the optical devices need be mounted so as to rotated relative to the fibrous material web. This is because two or more measurements can be performed at different relative orientations between the fibrous material web. Such a design allows for the detection of the fiber orientation.
In many of the embodiments, it is preferred that the source side polarizing filter be oriented such that the polarization direction of the linearly polarized radiation produced runs which are parallel to the travel or running direction of the fibrous material web. The sensor side polarizing filters can be either rotatably mounted so that they rotated opposite to one another. Alternatively, these sensor side filters can be non-rotatably mounted and oriented in a symmetrical manner relative to the web travel direction. Thus, for example, two sensor side filters can be provided, each of which has a sensor associated with it, which can rotated in opposite directions, for example, by approximately 30xc2x0 or 45xc2x0 relative to the web travel direction. The advantage of this arrangement is that there are minimal moving parts, which thereby avoids wear problems.
In general, it is possible to detect the fiber orientation with radiation of a single wavelength using the invention. However, since the interaction between the fibrous material web and the radiation is a function not only of the fiber orientation itself and additional factors such as the fiber length, fiber type and additional constituents and properties of the fibrous material web, but also of the wavelength of the radiation used for the measurement, more meaningful results can be obtained using several different wavelengths.
The invention also contemplates the use of individual radiation sources, each of which emits radiation at a specific wavelength, to be activated one after the other. In this case, a sensor of comparatively simple design, for example, a photodiode, can be used. This sensor would provide a signal representing the intensity thereof for each incident radiation.
On the other hand, it is preferred that a source which emits a discrete and/or continuous wavelength spectrum be used. This design makes it is possible to work simultaneously with different wavelengths. In this case, a sensor serving as a spectrometer can be used. The sensor is capable of detecting the intensity of the incident radiation separately by wavelength in order thus to be able to evaluate the signals separately by wavelength.
Moreover, the invention also provides that at least one source of electromagnetic radiation is arranged on one side of the fibrous material web. On the other side of the web is disposed at least one sensor for sensing the radiation emitted by the source and penetrating the fibrous material web. A polarizing filter is positioned between the source and the sensor device for influencing the propagation of the radiation as a function of its polarization characteristics. This filter may be located on either side of the web so that it is between the web and the source or between the web and sensor device. Additionally, the filter may be located on both sides of the web.
In this process, the fiber orientation is preferably detected in a moving fibrous material web, particularly a paper web moving at normal speed in a papermaking machine.
Such an on-line measurement of the fiber orientation makes it possible to intervene in the manufacturing process for the fibrous material web immediately after detecting a deviation from the desired fiber orientation and thus to create a rapid control loop.
According to one aspect of the invention, there is provided a system for determining the orientation of fibers in a fibrous material web, the system including at least one source of electromagnetic radiation disposed on one side of the fibrous material web, at least one sensor for sensing the electromagnetic radiation emitted by the at least one source disposed on another side of the fibrous material web, and at least one optical device disposed between the at least one source and the at least one sensor, wherein the electromagnetic radiation travels through the at least one optical device and the fibrous material web such that the at least one optical device influences a propagation of the electromagnetic radiation as a function of its polarization properties. The fibrous material web may be a paper web. The at least one optical device may provide for the transmission of linearly polarized radiation. The at least one optical device may be a polarizing filter. The polarizing filter may be rotatably mounted about an axis. The axis may be approximately perpendicular to a running direction of the fibrous material web. The at least one optical device may include at least two optical devices, one optical device being disposed on one side of the fibrous material web and another optical device being disposed on another side of the fibrous material web. The at least one optical device may comprise at least two optical devices, the at least two optical devices being disposed on one side of the fibrous material web. The at least two optical devices may be disposed between the at least one sensor and the fibrous material web. Each of the at least two optical devices may have a different orientation relative to a running direction of the fibrous material web.
The system may further comprise at least one optical device disposed between the at least one source and the fibrous material web. The at least two optical devices may be oriented symmetrically relative to the at least one optical device. Each of the at least two optical devices may be rotatably mounted about an axis. Each of the at least two optical device may be rotatable in opposite directions from one another. The at least one optical device may comprise a single optical device disposed between the at least one sensor and the fibrous material web. The electromagnetic radiation emitted by the at least one source may be polarized before it passes through the fibrous material web. The single optical device may be rotatably mounted. The at least one optical device may comprise a single optical device disposed between the at least one source and the fibrous material web. The electromagnetic radiation sensed by the at least one sensor may pass through the fibrous material web without being polarized. The single optical device may be rotatably mounted. The at least one sensor may comprise at least two sensors, each of the sensors being associated an optical device. The electromagnetic radiation may comprise one of a discrete and a continuous wavelength spectrum. The electromagnetic radiation may comprise a discrete and a continuous wavelength spectrum. The electromagnetic radiation may comprise one of visible light and infrared radiation. The electromagnetic radiation may comprise visible light and infrared radiation. The at least one sensor may comprise one of a spectrometer and a photodiode. The at least one sensor may be coupled to an analysis unit.
According to another aspect of the invention, there is provided a method for determining the orientation of fibers in a fibrous material web, the method including exposing a first side of the fibrous material web to electromagnetic radiation from at least one source, allowing the electromagnetic radiation to penetrate to a second side of the fibrous material web, influencing a propagation of the electromagnetic radiation as a function of its polarization properties with at least one optical device disposed between the at least one source and at least one sensor, and sensing the electromagnetic radiation on the second side with the at least one sensor. The fibrous material web may be a paper web. The at least optical device may comprise a polarizing filter disposed between the at least one source and the at least one sensor. The influencing may further comprise disposing a first optical device on the first side and a second optical device on the second side. The influencing may further comprise rotating the first optical device about an axis.
The second optical device may comprise a plurality optical devices. The plurality of optical devices may be arranged adjacent one another, each of the plurality being oriented to influence the propagation differently. The influencing may further comprise rotating the second optical device about an axis. The influencing may further comprise continuously moving the fibrous material wed between the first optical device and the second optical device. The influencing may further comprise continuously moving the fibrous material wed between the at least one source and the at least one sensor.
The method may further comprise analyzing a signal generated by the at least one sensor using an analyzer which is coupled to the at least one sensor. The method may further comprise analyzing the electromagnetic radiation after sensing. The analyzing may further comprise analyzing the electromagnetic radiation separately by wavelength. The exposing may comprise using a plurality of different sources. The different sources may vary a wavelength of the electromagnetic radiation over time. The exposing may comprise varying a wavelength of the electromagnetic radiation over time. The method may further comprise analyzing a signal generated by the at least one sensor using an analyzer which is coupled to the at least one sensor to determine one of a difference signal, a summation signal and a ratio signal. The method may further comprise analyzing signals generated by a plurality of sensors using an analyzer which is coupled to the plurality and determining one of a difference signal, a summation signal and a ratio signal. The determining may comprise determining a difference signal, a summation signal and a ratio signal. The method may further comprise moving the fibrous material web between the at least one source and the at least one sensor. The moving may be at a constant speed. The fibrous material web may be a paper web.
The electromagnetic radiation may comprises one of a visible light and a infrared radiation. The influencing may comprise first polarizing the electromagnetic radiation on the first side using a first polarizing filter and second polarizing the electromagnetic radiation on the second side using a second polarizing filter. One of the first polarizing filter and the second polarizing filter may be rotatable. The first polarizing may comprise using a rotatable first polarizing filter. The second polarizing may comprise using a plurality of oriented second polarizing filters. Each of the plurality of second polarizing filters may be rotatable.